If you let anyone upvote, it will be spammed by alts.
And just like on crap sites like Reddit, up/down votes will become meaningless. Merit carries a lot more weight.
By the way, OP, this has been discussed before and was rejected. We just don't need anything in addition to the merit system. If you like a post and have some sMerit to give, give it out. If the post is objectively high-quality, someone will probably merit it themselves if you don't have any sMerits. It's not a big deal.